A radial tire comprises in a known way a tread, two inextensible beads, two sidewalls connecting the beads to the tread and a belt or crown reinforcement positioned circumferentially between the carcass reinforcement and the tread. The carcass and/or crown reinforcement is composed, in a known way, of at least one ply (or “layer”) of rubber reinforced with reinforcer elements, such as cords, generally of the metal type in the case of tires for industrial vehicles carrying heavy loads.
Use is generally made, for the reinforcing of carcass and/or crown reinforcements, of single-strand metal cords composed of a central layer or core and of one or more layers of concentric threads positioned around this core. The most widely used three-layer cords are essentially cords of M+N+P construction, formed of a core of M thread(s), M varying from 1 to 4, surrounded by an intermediate layer of N threads, N typically varying from 3 to 12, itself surrounded by an external layer of P threads, P typically varying from 8 to 20, it being possible for the assembly to be optionally wrapped by an external wrapping wire wound helically around the external layer. Use is also made of multistrand metal ropes comprising several strands, as described above.
In a well-known way, these metal cords are subjected, in particular in the case of the carcass reinforcement, to high stresses during the running of the tires, in particular to repeated bending actions or variations in curvature, resulting in rubbing actions at the threads, in particular as a result of the contacts between adjacent layers, and thus in wear, and also in fatigue; they thus have to exhibit high resistance to the “fretting fatigue” phenomena.
As regards the crown reinforcement, a tire of a heavy industrial vehicle, in particular a civil engineering vehicle, is subjected to numerous attacks. Specifically, this type of tire usually runs on an uneven road surface, sometimes resulting in perforations of the tread. These perforations allow the entry of corrosive agents, for example air and water, which oxidize the metal reinforcer elements of the crown reinforcement, in particular crown plies, and considerably reduce the lifetime of the tire.
In addition, it is particularly important for the reinforcer elements to be impregnated as much as possible with rubber and for this material to penetrate into all the spaces located between the threads and/or the strands constituting the cords. This is because, if this penetration is inadequate, empty channels or capillaries are then formed, along and inside the cords, and the corrosive agents, such as water or even oxygen of the air, liable to penetrate into the tires, for example as a result of cuts to their treads, make their way along these empty channels. The presence of this moisture plays an important role by bringing about corrosion and by accelerating the degradation processes above (phenomena referred to as “corrosion fatigue” and crown attack), in comparison with use in a dry atmosphere.
All these phenomena of fatigue and of attacks are the cause of a progressive deterioration in the mechanical properties of the cords and can affect, for the most severe running conditions, the lifetime of the cords.
In order to overcome the above disadvantages, Application WO 2005/071157 provided three-layer cords of 1+M+N construction, in particular of 1+6+12 construction, one of the essential characteristics of which is that a sheath composed of a rubber composition, referred to as rubberizing composition, covers at least the intermediate layer composed of the M threads, it being possible for the core (or individual thread) of the cord to be itself covered or not covered with rubber. By virtue of this specific architecture, the cord exhibits excellent properties of endurance in fretting fatigue and of resistance to attacks which are in particular improved with respect to the cords of the prior art. The longevity of the tires and that of their carcass and/or crown reinforcements are thus very substantially improved.
However, despite the presence of the rubberizing composition between the threads, during the use of the tire, corrosive agents, for example water, can penetrate into the reinforcements, in contact with the metal reinforcer elements, and can corrode them via external strands and/or threads, thus rapidly degrading their mechanical properties and properties of adhesion to the rubber composition adjacent to these metal reinforcer elements, known as calendering composition.
The use of a corrosion inhibitor makes it possible, on the one hand, to prevent the action of the corrosive agents by virtue of the formation of a protective film around the metal reinforcer element and, on the other hand, by adsorption on the metal reinforcer element, to slow down, indeed even to halt, the corrosive action of the corrosive agents on and within the metal reinforcer element.
A composition comprising a derivative of the family of the triazines as corrosion inhibitor is known from the document JP05177772. However, such a compound is relatively expensive. Furthermore, it is desirable to limit as much as possible the amount to be employed of compounds which may have an environmental impact.